Dosing of acids or bases to attain a desired pH (acidification and neutralization)

Successive Combination of Reaction Processes

The ability to save the output solution after each chemical reaction allows the simulation of whole chains of water treatment processes. An example is shown in the right screenshot.

The idea is simple. You save the output solution under a suitable name and use it as input water for a subsequent process: addition of chemicals, mixing of waters, mineral dissolution, CO2 gas exchange and/or neutralization to attain some specific target pH – as presented below.

Dosing of Chemicals to Attain a Desired pH

The reaction module named “Dosage of chemicals to attain a target pH”1
calculates the required amount of a chosen reactant – acid or base – to achieve a target pH:2

The chosen option affects the required amount of chemicals – especially when neutralization of acidic solutions is performed.

Example 1. An acid mine water (example file mine.sol) should be neutralized from pH 3.29 to 7 by addition of Ca(OH)2. The corresponding input panel is shown on the right. In this first example, mineral precipitation is disabled.

Click on Start to run the calculation. The results are displayed in the blue panel on the right-hand side:

7.77 mmol/L Ca(OH)2 is required to increase the solution’s pH from 3.29 to 7.00.

But: During the neutralization minerals precipitate, which decrease the pH again. In this example, the calculation predicts for the entire process, i.e. reaction plus precipitation, a final pH value of no more than 5.79.

Example 2. Let’s re-run Example 1 with a subtle but important modification: we activate the checkbox ‘Allow precipitation of minerals’ in the above panel.3

The results are displayed on the right-hand side:

11.20 mmol/L Ca(OH)2 is required to increase the solution’s pH from 3.29 to 7.00 under the assumption that minerals precipitate.

As shown in the scheme, dosing 11.20 mmol/L Ca(OH)2 would over-neutralize the solution to pH 8.90 “just before” minerals precipitate. It’s the precipitation which brings the pH to our desired target-pH.

Irrespective of the fact that iron precipitates as siderite and amorphous Fe(OH)3 the neutralized solution still contains small amounts of dissolved Fe. A complete removal of Fe requires oxidizing conditions. This will be shown in Example 3.

Example 3. Now we repeat Example 2 by maintaining a high oxidation potential of pe 11 via O2 exchange. For this purpose click on button Setup and activate the checkbox “Open Redox System”.

The results are displayed on the right-hand side:

15.16 mmol/L Ca(OH)2 is required to increase the solution’s pH from 3.29 to 7.00 under the assumption of enhanced mineral precipitate (forced by aeration).

In this case the neutralized solution is free of any dissolved Fe. This is due to the complete precipitation of iron(III) as amorphous Fe(OH)3.4

Note. In all examples, the neutralized solution (with or without mineral precipitation) can be saved and used as input water for subsequent reaction processes.

Remarks & Footnotes

This module is available in the upper menu bar ‘Extras’, or by the shortcut Alt+P. ↩

If you select a too-weak acid (or a too-weak base) to enforce large pH changes in a well buffered solution the program will prompt a message that it cannot provide a numerical solution (because it’s chemically impossible). ↩